Field of the Invention
The present invention relates to a semiconductor device and a method for its fabrication.
Though applicable to all semiconductor devices, and specifically to semiconductor devices for power and logic circuits, the present invention and the problems on which it is based are explained with regard to a PMOS transistor for a charge pump circuit in a p-type semiconductor substrate made of silicon.
In the case of customary PMOS transistors using twin CMOS well process technology with the p-type semiconductor substrate, the n-type well in which a transistor is situated cannot be biased negatively with respect to the p-type substrate because an interfering substrate current would flow via a relevant forward-biased pn junction.
The same applies correspondingly in the case of NMOS transistors using triple CMOS well process technologies with a p-type semiconductor substrate, where a further p-type well is additionally provided in the n-type well, or for the respective conductivity types with an n-type semiconductor substrate.
The negative source voltage customarily used in the case of such PMOS transistors effects an increase in the threshold voltage as a result of the so-called substrate control effect.
This effect manifests itself disruptively, for example, in the design of the customary charge pump circuit used for the on-chip generation of an increased voltage of, typically, 10 to 15 V from a supply voltage of typically 5 V.
On account of the problems outlined below, the effectiveness of the charge pump circuit diminishes as the threshold voltage of the NMOS transistors provided in a diode connection increases.
In the case of the charge pumps, the increased threshold voltage is intrinsically accepted and specific countermeasures are implemented in order to overcome the problem. In the case of a PMOS charge pump circuit for -12 V, for example, a boost circuit for increasing the gate voltage and also a correspondingly adapted number of pump stages are provided in order to compensate for the reduced effectiveness due to the substrate control of the PMOS transistors. However, solutions of this type are complicated in terms of circuitry.